[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

L44J. Pety et al.: Deuterium fractionation in the Horsehead edgephotoelectric heating, to the coldest and shielded gas wherestrong deuterium fractionation is taking place. Therefore, theHorsehead edge is the kind of source needed to serve as a referencefor PDR models (Pety et al. 2006) and offers a realistictemplate to analyze more complex galactic or extragalacticsources.Acknowledgements. We thank M. Guelin for useful comments and theIRAM PdBI and 30 m staff for their support during the observations. J.R.G. wassupported by an individual Marie Curie fellowship, contract MEIF-CT-2005-515340.tel-00726959, version 1 - 31 Aug 2012Fig. 3. Chemical models for different minimum gas temperatures: 15,20, 30 and 60 K. The density is n H = 4 × 10 5 cm −3 and the illuminatingradiation field is χ = 60. Temperature profiles and predicted[H 2 D + ]/[H + 3 ]and[DCO+ ]/[HCO + ] abundance ratios are shown as afunction of A V .The[DCO + ]/[HCO + ] ratios inferred from observationsin the cold condensation at δx ∼ 40−45 ′′ and in the warm PDR gas atδx ∼ 10−15 ′′ are shown respectively with the blue and red arrows.Caselli et al. 1999). The C 18 O J = 2−1 emission shown in Fig. 2substantially decreases at the DCO + peak. This behavior is reminiscentof CO depletion but it could also come from a combinationof lower excitation and of opacity effects. Future observationsof molecular tracers of gas depletion are needed toconstrain the dominant scenario.The DCO + lines stay undetected in the warm gas whereHCO + (not shown here) and H 13 CO + still emit. Indeed, DCO +can not be abundant in the photodissociation front, where thelarge photoelectric heating rate implies warm temperatures(T k > 50 K), because the reaction of H 2 D + with H 2 dominatesand implies a low H 2 D + abundance ([H 2 D + ]/[H + 3 ] ≃2 × 10 −4 ). From the upper limit of the DCO + emission atδx = 10−15 ′′ (A V ∼ 1), we estimate a low abundance ratio[DCO + ]/[HCO + ] < 10 −3 in the far-UV photodominated gas, inagreement with the model predictions.The small distance to the Horsehead nebula (∼400 pc),its low FUV illumination and its high gas density imply thatmany physical and chemical processes, with typical gradientlengthscales ranging between 1 ′′ and 10 ′′ , can be probed in asmall field-of-view (less than 50 ′′ ). The Horsehead edge thusoffers the opportunity to study in great detail the transition fromthe warmest gas, dominated by photodissociation processes andReferencesAbergel, A., Teyssier, D., Bernard, J. P., et al. 2003, A&A, 410, 577Brown, P. D., & Millar, T. J. 1989, MNRAS, 237, 661Caselli, P., Walmsley, C. M., Tafalla, M., Dore, L., & Myers, P. C. 1999, ApJ,523, L165Cernicharo, J., & Guelin, M. 1987, A&A, 176, 299Flower, D. R. 1999, MNRAS, 305, 651Flower, D. R., Pineau Des Forêts, G., & Walmsley, C. M. 2006, A&A, 449, 621Gerlich, D., Herbst, E., & Roueff, E. 2002, Planet. Space Sci., 50, 1275Goicoechea, J. R. 2003, Ph.D. Thesis, Universidad Autonoma de MadridGoicoechea, J. R., Pety, J., Gerin, M., et al. 2006, A&A, 456, 565Guelin, M., Langer, W. D., & Wilson, R. W. 1982, A&A, 107, 107Guilloteau, S., Piétu, V., Dutrey, A., & Guélin, M. 2006, A&A, 448, L5Habart, E., Abergel, A., Walmsley, C. M., Teyssier, D., & Pety, J. 2005, A&A,437, 177Hatchell, J., Millar, T. J., & Rodgers, S. D. 1998, A&A, 332, 695Hily-Blant, P., Teyssier, D., Philipp, S., & Güsten, R. 2005, A&A, 440, 909Le Bourlot, J., Pineau Des Forets, G., Roueff, E., & Flower, D. R. 1993, A&A,267, 233Le Petit, F., Nehmé, C., Le Bourlot, J., & Roueff, E. 2006, ApJS, 164, 506Leurini, S., Rolffs, R., Thorwirth, S., et al. 2006, A&A, 454, L47Linsky, J. L., Draine, B. T., Moos, H. W., et al. 2006, ApJ, 647, 1106Milam,S.N.,Savage,C.,Brewster,M.A.,Ziurys,L.M.,&Wyckoff, S. 2005,ApJ, 634, 1126Penzias, A. A., & Burrus, C. A. 1973, ARA&A, 11, 51Pety, J. 2005, in SF2A-2005: Semaine de l’Astrophysique Française, ed.F. Casoli, T. Contini, J. M. Hameury, & L. Pagani, 721Pety, J., Goicoechea, J. R., Gerin, M., et al. 2006, in SF2A-2006: Semaine del’Astrophysique Française [arXiv:astro-ph/0612588]Pety, J., Teyssier, D., Fossé, D., et al. 2005, A&A, 435, 885Philipp, S. D., Lis, D. C., Güsten, R., et al. 2006, A&A, 454, 213Roberts, H., & Millar, T. J. 2000a, A&A, 364, 780Roberts, H., & Millar, T. J. 2000b, A&A, 361, 388Roueff, E., Tiné, S., Coudert, L. H., et al. 2000, A&A, 354, L63Smith, D., Adams, N. G., & Alge, E. 1982, ApJ, 263, 123Solomon, P. M., & Woolf, N. J. 1973, ApJ, 180, L89Turner, B. E. 1990, ApJ, 362, L29van Dishoeck, E. F., Thi, W.-F., & van Zadelhoff, G.-J. 2003, A&A, 400, L1Walmsley, C. M., Flower, D. R., & Pineau des Forêts, G. 2004, A&A, 418, 1035Ward-Thompson, D., Nutter, D., Bontemps, S., Whitworth, A., & Attwood, R.2006, MNRAS, 369, 1201Watson, W. D. 1974, ApJ, 188, 35